Hot melt adhesives containing isocyanate compounds have been common in a variety of adhesives used in construction applications for many years. Thermal curing and moisture curing isocyanate containing adhesives have been used in laminate construction, automobile construction, appliance construction, etc., in bonding such materials as woods, plastics, metals, composites, paper, paper board, film, etc. The major benefit of the use of isocyanate containing hot melt adhesives is the mechanical stability of the cured adhesive bond. Such bonds are formed by chain extension and cross-linking caused by the reaction between the active isocyanate groups and reactive adhesive components. A large number of types of isocyanate containing hot melt adhesives are known. Such adhesives can either contain free isocyanate compounds or can contain isocyanate compound bonded in preformed materials such as isocyanate terminated pre-polymers.
In the dispensing and application of molten hot melt adhesive to a use locus, the adhesive is commonly heated to an elevated temperature sufficient to cause the adhesive to melt and to attain a viscosity compatible with dispensing in common hot melt dispensing equipment. Temperatures ranging from at least 150.degree. F. up to 325.degree. F. are common. Such temperatures cause the adhesive, once applied, to develop a bond rapidly, often within 100-200 seconds. Most commonly, hot melt isocyanate containing adhesives are used with heated platen melters that dispense the adhesive from cylindrical containers such as five gallon pails, ten gallon steel containers, fifty-five gallon drums, etc. Currently this equipment consists of an apparatus which extrudes the adhesive from the pail. A heated platen contacts adhesive in the pail, heats the surface of the adhesive until molten, wherein a hose installed in the platen removes the heated molten adhesive to an extruder nozzle which applies the adhesive to a use locus. Current manufacturers of heated platen type isocyanate dispensing equipment are Nordson, model number 506, and Mercer, model number 1030 HS.
In the use of such dispensing equipment a heated platen is inserted into the open top of a pail. Since such pails have sealed bottoms, the heated platen effectively dispenses the contents of the container to within approximately one inch from the bottom of the containers to the use locus. However, when the heated platen is removed from the pail, large proportions of isocyanate compounds can be released into the atmosphere. The atmospheric isocyanate compound is released as a vapor, aerosol or particulate that can contact workers' skin surface and can be breathed. In addition, a "plug" of adhesive is left at the bottom of the container. Landfill regulations in many States require that this isocyanate containing adhesive be disposed of as hazardous waste.
Breathing air in which isocyanate exceeds the threshold limit values as established by the American Conference of Governmental Industrial Hygienist can cause respiratory distress. Skin or eye contact with uncured materials can also result in adverse reactions. The risk of exposure to excess levels of isocyanate is particularly high when a pail or drum has been emptied and must be changed. Therefore, the use of standard containers and heated platen equipment must be done in a controlled environment, wherein the atmosphere is rapidly exchanged or workers are otherwise protected from contact with isocyanate vapors, aerosols or particulates.
Because of the toxic nature of isocyanate vapors, fumes or particulates, a substantial need has existed for many years to develop a system for dispensing isocyanate containing hot melt adhesives that permit the efficient melting and unloading of containers filled with these adhesives and application of these adhesives to the use locus, without generating toxic concentrations of the isocyanate material within the air. Currently, a concentration of 20 parts per billion (ppb) isocyanate in the air is considered to be the maximum allowable concentration in common industrial environments. These limits apply to 4,4'-diphenyl methane diisocyanate, the most commonly used isocyanate in isocyanate containing hot melts. To the best of our knowledge, no commercially successful container or dispensing equipment has been developed in the prior art which can substantially eliminate toxic concentrations of isocyanate in the work place.